This invention relates to a frequency conversion system and more particularly to a system for converting the frequency of a musical tone signal obtained by electrically picking up mechanical vibration of a musical instrument.
In addition to an electronic musical instrument such as an electronic organ for producing tone signals through electronic circuits, an electric musical instrument such as an electric piano and electric guitar is also widely used to electrically pick up mechanical vibration of vibration pieces or strings of the musical instrument to produce tone signals. The electric piano, together with the electric guitar etc., is often used in light music concerts frequently held at local or provincial areas and for this reason it is preferred that it be compact enough for a musician to carry. A compact electric piano can be attained by reducing the number of strings and keys, but this reduces the number of sound sources, narrowing the compass of the electric piano. An octave conversion system may be used to cover a wider compass in a compact electric piano. This system is used to further lower by octave or octaves, as required, the frequency of tone signals belonging to the lowest octave of the electric piano. The use of the octave conversion system permits a compact keyboard and/or a compact frame for supporting mechanical vibration pieces or strings without narrowing the compass of the musical instrument. In this case, since tone signals of different frequencies can be obtained from a common string or different strings of equal size, a compact keyboard and/or compact frame can be used.
As a frequency or octave conversion system a so-called octave box is conventionally used, in which original tone signals obtained by electrically detecting the mechanical vibrations of the strings are supplied to a lowpass filter to provide a sinusoidal wave signal of the fundamental frequency of the tone signal, and such a sinusoidal wave signal is converted by a clipper circuit to a rectangular wave signal. The rectangular wave signal is frequency-divided by a factor of 2, for example, and the frequency-divided rectangular wave is filtered out by a lowpass filter so as to produce a sinusoidal wave signal. The envelope of the original tone signal is extracted by an envelope detector to produce an envelope signal. The sinusoidal wave signal having a frequency of one half of the fundamental frequency of the tone signal is furnished with the envelope by a voltage-controlled amplifier responsive to the envelope signal. The output signal of the voltage-controlled amplifier and original tone signal are mixed by a mixer so as to produce an one-octave lower tone signal. The harmonic components of the one-octave lower tone signal consists of the fundamental frequency component of the original tone signal and its harmonic components.
Such frequency or octave conversion system is complicated in its arrangement and it is difficult to accurately extract the envelope of the original tone signals over the whole range of the fundamental frequency. That is, it is difficult to effect a frequency conversion without changing the original envelope.